C. trachomatis variants have been described that are incapable of undergoing inclusion fusion in cell culture. These variants represent 1-2 percent of clinical isolates in Seattle and have been defined as lacking detectable IncA, a chlamydial protein that localizes to the inclusion membrane. Twenty-six independent incA mutant isolates have been sequenced and organized into several distinct categories. The overall goal of the proposed research is to identify distinctions between wild type and non-fusogenic strains and to exploit differences that are defined to better understand chlamydial development and pathogenesis. Experiments are planned to examine molecular and cell biology as well as clinical manifestations of the mutant chlamydiae. Each variant will be compared with matched wild-type controls. In the first aim molecular analyses will be done to determine mechanisms responsible for loss of expression of IncA and possibly other Inc's. In the second aim, growth and development of non-fusogenic strains will be studied in cell culture models. Finally, in the third aim the clinical relevance of the non-fusogenic phenotype will be determined using a retrospective case-control analysis and a monkey model of chlamydial infection. Study of these natural mutants will lead to a better understanding of chlamydial growth, development and pathogenesis.